A number of inventions in the past related to reforming processes in the petrochemical industry has led to significant process efficiency improvements. One such example is the development of large pore zeolite catalysts, doped with specific metals, rendering catalysts with a high selectivity suitable for precision reforming and/or synthesis, which for example has made possible more effective and economic production of a range of highly demanded commercial liquids based on hydrocarbon feedstocks. However, the catalysts were soon discovered to be sensitive to sulfur poisoning, leading to techniques to desulfurize the hydrocarbon feed being developed. Later, such catalysts were also found to be quickly deactivated by water, thus corresponding protecting technologies to lower the water content in the process gas streams were also developed.
In turn, the low-sulfur and low-water conditions led to coke formation and plugging within reactor systems, an effect later possible to relate back to a severe form of disintegrating attack on metallic materials of the equipment parts, like furnace tubes, piping, reactor walls, etc. This metal disintegrating mechanism was actually already known since the 1940's as “metal dusting” however, this phenomenon was seldom seen because at the time reforming techniques included high sulfur levels in the process gas and very high reforming and synthesis pressures, (since less effective catalysts were available).
Thus, with the above description of the historic developments as a background, it is understood that, in the petrochemical industry today, there is a need for a solution against the effects, of and the cause for, metal dusting.
As earlier mentioned, metal dusting is a form of carburization where the metal disintegrates rapidly into coke and pure metal. The dusting metal particulates can be transported with the process gas, accumulates downstream on various reactor parts, and throughout the whole reactor system, metastasize catalytic coking that can create blockage.
It is generally appreciated that metal dusting is a large concern in the production of hydrogen and syngas (H2/CO mixtures). In these plants, methane and various other higher hydrocarbons are reformed or partially oxidized to produce hydrogen and carbon monoxide in various amounts for use in producing other higher molecular-weight organic compounds. Increased reaction and heat-recovery efficiencies of the processes necessitate operating process equipment at conditions that favor metal dusting.
The need for increased heat recovery in ammonia-synthesis processes has caused metal dusting problems in the heat-recovery section of the reformed-gas system, as well as in the reformer itself.
Metal dusting is also a problem in direct iron-ore reduction plants wherein reformed methane is dried and reheated to enhance ore-reduction efficiencies. Metal dusting occurs in the reformer, reformed-gas reheater and piping up-stream of the ore-reduction system. Metal dusting is also experienced in the heat-treating industry in equipment that handles items being treated (annealed, carburized, etc.). Gases used in heat treating mix with oil residue to form gases that are chemically favorable for metal dusting. Gas mixtures used for carburizing can also cause metal dusting if control of the chemistry of the process is not managed.
Petroleum refineries experience metal dusting in processes involving hydro-dealkylation and catalyst regeneration systems of “plat-former” units.
Other processes wherein metal dusting occurs are nuclear plants that employ carbon dioxide for cooling, the recycle-gas loop equipment of coal-gasification units, in fired heaters handling hydrocarbons at elevated temperatures, ironmaking blast furnaces in steel mills, and fuel cells using molten salts and hydrocarbons.
In recent years, there has been an emphasis on reforming and synthesis technology developments to make possible commercialization of remotely located, so called “stranded gas reserves”. The synthesis step, based on further developments of the Fischer Tropsch process, will require the use of compositions of the synthesis gas that will cause severe metal dusting, with lower steam to carbon ratios and higher CO/CO2 ratios. However, only small steps this direction have been taken due to lack of material with sufficient resistance to metal dusting.
Other solutions used today to provide protection against metal dusting and reduce coke formation, are the use of advanced nickel or iron base alloys with high amounts of chromium and certain additions of aluminum. Some surface modification methods based on diffusion techniques or coatings through overlay welding, laser-fusion, Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD) or spraying has also been tested. Many of these methods involve materials based on transition metals, such as iron, nickel and cobalt, which are known for their catalytic properties that promote coke formation.
There are metals, such as Cu and Sn, that are known to be resistant or immune to carburization and coke formation, but have either a melting point, which is too low or insufficient oxidation resistance. Oxidation resistance is required since the solid coke is periodically removed by oxidation in steam and air. Consequently, the metal surfaces in contact with the carburizing process gas must also have adequate oxidation resistance, which excludes Cu and low alloyed Cu as a useful carburization-resistant material in practice. Even if the decoking step can be excluded in some processes, the start-up procedures after an inspection or other stops inevitably require oxidation-resistant metal surfaces.